Liquid crystal displays with polarized infrared illumination

ABSTRACT

Systems and methods for IR readable transmissive and reflective displays are disclosed that do not suffer from a mirror-like appearance or undesirable dimming of the display due to sequential stacks of polarizers. The disclosed systems and methods use available IR LEDs in addition to, or in place of, visible light LEDs. An illuminator or integrator, which is a lightguide, is designed to maintain the polarization state of the light. The display can use a regular visible light, front polarizer and hence does not suffer from brightness reduction caused by an IR capable polarizer.

FIELD OF THE INVENTION

The present disclosure relates to infrared readable liquid crystaldisplays, and more specifically, to illuminating liquid crystal displayswith polarized infrared light to extend readability beyond the usablecontrast range of the liquid crystal display polarizers to longerwavelengths.

BACKGROUND OF THE INVENTION

Modern liquid crystal displays (LCDs) can comprise millions ofindividual pixels, which in an LCD is a thin multilayered structure ofmany components. Pixels in an LCD are illuminated using an unpolarizedlight source, such as light emitting diodes (LEDs).

Most LCD pixels are based on two functional principles: 1) anelectrically controllable liquid crystal layer between transparentsubstrates changes the polarization state of the light passing throughit based on applied electrical signals and 2) one or more polarizers andoptional additional optical films transform a difference of polarizationstates into visible bright and dark contrast regions. Together thepolarizer(s) and the liquid crystal layer form an electricallycontrollable light valve, which lets a portion of the light pass throughdepending on the electrical stimulus.

Since most LCDs are flat or slightly curved and may have a substantialsize, generally film polarizers are required. Such films are adhered toone or more surfaces of the display and cover substantially the entireimage area. Film polarizers typically comprise two layers of atransparent isotropic polymer such as cellulose Tri-Acetate (TAC),sandwiching a stretched Poly Vinyl-Alcohol (PVA) layer containinganisotropic chromophores, which align due to the stretching of the PVAand provide the polarization effect.

The chromophores are typically iodine, which is present in the PVA asI₃- and I₅-PVA complexes. Alternatively, the PVA layer may beimpregnated with different anisotropic dye molecules that together coverthe visible range of the spectrum. Common to either the iodine method orthe dye method is that the range of usable contrast is limited to thevisible range of the spectrum as the PVA-iodine complexes and thetypically used anisotropic dyes are transparent in the near infrared.Hence, when viewing normal LCD's with IR sensitive cameras or nightvision equipment, they do not show an image when operated undernon-visible IR-illumination only.

It is sometimes desirable or even necessary that LCDs can be viewedusing invisible infrared light. For example, it may be necessary to readan LCD with night vision goggles in the absence of any visible light, orit may be necessary to read LCDs with infrared cameras.

Common to most outdoor optical equipment is that it selectively usesvertically polarized light to eliminate glare from glancing reflectionsof smooth or wet surfaces. This is achieved by adding polarizationelements to the front of the camera lenses. Such polarizing elements canbe selected so that they can reject visible and/or IR light with anundesired polarization state.

While it is possible to add additional types of dye molecules that areabsorbing at longer infrared wavelengths to the chromophore layer of apolarizer to extend the polarizer's usable contrast range, this isundesirable as such polarizers necessarily have a lower transmission inthe visible range, leading to darker displays. This is because addingmore dye molecules reduces transmission, and most dyes have higher orderabsorption bands. For example, a dye that would absorb at 900 nm in theinfrared range would likely also absorb at around 450 nm, which is inthe visible range. A higher absorption (or lower transmission) is aproblem especially for reflective displays or battery-operated displayswhere the lower visible transmission cannot be compensated for with astronger light source.

Alternate types of polarizers such as wire grid polarizers, cholestericfilm polarizers, and multilayer birefringent stack polarizers functionon the principle of transmitting one polarization state while reflectingthe other. These types of polarizers are an option for the rear side ofa display, if backed by a suitable absorber for the transmittedpolarization. Such polarizers typically have good contrast in the nearinfrared as they are not based on absorption of dyes. For example, acommon 3M DBEF polarizer or a Nagase WGF works well to 850 nm. Suchdevices can be used instead of absorptive rear polarizers in an LCD. Forexample, an IR wire grid polarizer can be placed behind a display as therear polarizer. However, it is undesirable to use such polarizers on thefront of a display as they have a metallic, mirrorlike appearance due tothe specular reflection of about half the incoming light. If they areused in front of the display, they will have to be hidden under anadditional absorptive polarizer that will remove the reflected portionof the light in the visible range, while taking appropriate steps thatsuch specular reflection of infrared light will not interfere withreading the display with infrared equipment.

Thus, it would be desirable to extend the usable contrast range of LCDsto longer wavelength, and to make them readable by infrared equipmentwithout the side-effects of lower brightness in the visible range,mirrorlike appearance, or the need of additional polarizers that preventthe mirrorlike appearance.

BRIEF SUMMARY OF THE INVENTION

For purposes of summarizing the invention, certain aspects, advantages,and novel features of the invention have been described herein. It is tobe understood that not necessarily all such advantages may be achievedin accordance with any one particular embodiment of the invention. Thus,the invention may be embodied or carried out in a manner that achievesor optimizes one advantage or group of advantages as taught hereinwithout necessarily achieving other advantages as may be taught orsuggested herein.

Many liquid crystal displays have an integrated light source, either asa backlight or a front light. The front light or backlight typically iscomposed of the actual light source and an ‘integrator’, whichdistributes the light evenly across the display. The light sources aretypically CCFL tubes, electroluminescent films, or, most suitably, lightemitting diodes, such as either solid state LEDs or organic LEDs. Anintegrator may be a light cavity or a transparent lightguide thatdistributes the light evenly across the display surface.

Since LEDs are available, which emit light in the required range ofinfrared wavelength, such LEDs can replace or be added to visible lightLEDs to create an infrared illuminator. This invention and disclosurecomprise such an illuminator or integrator, which is a lightguide thatis designed to maintain the polarization state of the light. The displaycan use a regular, visible-light, front polarizer and hence does notsuffer from a brightness reduction caused by an IR-capable polarizer.

A polarizer in front of an LCD display serves two functions: 1) itpolarizes incoming light and 2) it analyzes, or turns into contrast, thepolarization state of the light coming out of the display. If theobserver uses polarized light, for example with polarized sunglasses or,more suitably, an optical device, such as a camera or night visionequipment, the second function is already provided by this equipment.Hence, to extend the usable wavelength range of an LCD display into theIR range, either both functions need to be extended into the IR range,or if it can be assured that any relevant IR equipment has its ownpolarizer, it is sufficient to only extend polarizing the incoming lightinto the IR range.

Visible range polarizers are isotropic and transparent in the infraredrange used by night vision equipment or infrared cameras, specificallyin the range of 700 to 1100 nm. Hence, if a display is illuminated withpolarized IR light it will remain readable to IR detection equipmentthat uses an IR-polarizer for glare reduction.

If a display has an infrared-capable, rear polarizer, illuminating itwith a polarized IR source will create contrast even without a polarizeron the optical equipment as light will be reflected out through thedisplay, depending on the polarization state after the first paththrough the display.

One exemplary embodiment of this invention and disclosure is an infraredtransmissive LCD display with a backlight comprising a lightguide andLED edge illumination. In addition to visible-light LEDs, IR-emittingLEDs are added to the edge of the lightguide. An infrared polarizer,such as a wire grid polarizer, is placed between the infrared andoptionally the visible LEDs and the lightguide edge to provide asuitable polarization state. The light guide is designed to maintainthat polarization.

In visible light operation polarized or unpolarized visible light isguided through the lightguide into the display and passes through thedisplay from the back to the front. When exiting the lightguide, thelight first encounters a polarizer that polarizes it into the desiredstate. It then passes through the liquid crystal layer, which can changethe polarization of the light depending on the desired state of a pixel.Finally, it passes through the front polarizer, which acts as theanalyzer and thus creates a brightness contrast, visible to the nakedeye.

In infrared operation, the IR LEDs emit light that is polarized whenpassing through the IR polarizer. The polarized IR light passes throughthe lightguide and is directed towards the LCD. The polarizer in the LCDappears transparent to the IR light. The liquid crystal layer changesthe polarization state if needed, based on the state of the pixel. Thefront polarizer appears transparent to the IR light and hence does notact as an analyzer.

As a result, some areas of the display emit light in one polarizationstate whereas other areas emit light in another polarization state.Infrared sensitive optical equipment such as infrared cameras or nightvision goggles with an IR antiglare polarizer can detect the image astheir polarizer acts as the analyzer, turning the polarizationdifferences into a brightness contrast for the sensor element. Humaneyes cannot detect differences in the polarization state of the light,but they can detect differences in brightness. The function of theanalyzer is to turn differences in polarization states into differencesin brightness by letting through one polarization state while absorbingor reflecting the other. This embodiment may also include furtheroptical films, such as retardation films, compensation films, or otherlight management films that optimize the performance of the device.

Another exemplary embodiment of this invention and disclosure is areflective LCD display with polarized IR illumination. In visible lightoperation, ambient light or light from visible light LEDs in the frontlight travels through a front light that leaves it unchanged, and thento the front polarizer where the light is polarized. In an alternateembodiment, the ambient visible light first encounters a visible light(absorptive) polarizer that imparts a specific polarization state, whichremains unchanged when traveling through the front light guide. Thepolarized light then enters the liquid crystal layer where itspolarization state may be changed depending on the electrical signalsapplied. It may then pass through an optional polarizer before beingreflected by a mirror, or it may be reflected by a polarizing mirrorsuch as a reflective polarizer, backed with an absorber.

On the return path, the light again passes through the liquid crystallayer and through the front polarizer, which acts as the analyzer,turning differences in polarization state into a brightness contrast.Finally, the light passes through the front light, which appears mostlytransparent to the light. In an alternate embodiment, the light firstencounters the front light guide where its polarization state remainsunchanged, before being analyzed in the front polarizer.

In IR light operation, IR light from the IR LEDs is coupled into thefront light lightguide via an IR polarizer. The polarized IR lightpasses through the lightguide, which may be positioned in front of thefront polarizer or between the front polarizer and the LCD withoutchange in polarization state. The lightguide sends the light through theLCD, where the polarization state of the light can be changed accordingto the signals applied. The light is selectively reflected at only onepolarization by a reflective polarizer or polarizer-mirror combinationbehind the display, while the other polarization is absorbed.

The reflected light returns through the liquid crystal layer, wherefurther polarization adjustment may happen. Upon exiting the LCD layer,the light encounters the front polarizer and front light, which bothappear transparent to the IR light. As a result, some areas of thedisplay appear to emit IR light, while others do not. Infrared sensitiveoptical equipment will detect different brightness levels depending onthe liquid crystal state even without an IR antiglare polarizingelement. This embodiment may also include further optical films such asretardation films, compensation films, diffuser films and other lightmanagement films that optimize the performance of the device.

Another exemplary embodiment of this invention and disclosure is areflective liquid crystal display for a digital license plateapplication comprising a reflective liquid crystal display with a frontlight, which includes polarized IR illumination. License platerecognition systems operate at specific infrared wavelengths, such as750 nm, 850 nm, 870 nm and others. The reflective LCD may be a bi-stableor multi-stable LCD due to the low power requirements of such displayscompared to displays requiring constant updating. One such bistable LCDtype may be a Memory-in-Pixel LCD, another may be a bistable nematic LCDknown as ‘Binem’ or a bistable nematic display known as ‘ZBD’.

The LCD may work on a single polarizer basis or have a reflective rearpolarizer, such as a multilayer polymer stack available from 3M™ knownas DBEF, a wire grid polarizer such as a Nagase WGF, or similar, whichhave usable contrast from about 380 nm to greater than 850 nm. The frontlightguide may be located in front of or behind the front polarizer. Thefront lightguide is illuminated from the edge with optional white lightLEDs for night visibility and with a plurality of IR LEDs selected for adesired wavelength or multiple desired wavelengths depending on therequirements of the location where such a license plate is issued. Forexample, a display may be fitted with several 750 nm and several 850 nmLEDs if that matches the requirement. Other combinations are possible aswell.

A narrow polarizer strip of a polarizer with good polarizationefficiency at the desired wavelengths is placed between the IR LEDs andthe lightguide. Such polarizer may be a dye type polarizer with dyeselected for infrared operation only and it may not have goodtransmission or polarization efficiency in the visible spectrum as novisible light is required to pass through it. Another suitable type ofpolarizer may be a wire grid type polarizer or multi-layer stackpolarizer as such polarizers are simpler and easier to produce at alower cost than wire grip polarizers for the visible range.

A polarization preserving lightguide may be made from transparentpolymers, glass, or a combination of different transparent materials andmay be coated with materials with different refractive indices. Thelight travels from the light source through the lightguide due to totalinternal reflection. Additional features such as certain shapes ofalternating materials or certain surface structures, such as dimples orprisms, cause the light to be sent towards the display, but not to theopposite surface.

The light passes through the liquid crystal layer where its polarizationmay get changed according to the liquid crystal alignment beforeencountering a selective reflection in the rear reflective polarizer. Indark areas the light has a polarization state that passes through thereflective polarizer and gets absorbed in a black layer placed behindthe display assembly. In bright areas, the light gets reflected backthrough the display, where further polarization changes may happen. Thefront polarizer appears transparent to infrared light.

These structures in the front light are designed to allow at least aportion of the light being reflected by the display to pass through tothe front surface. A license plate recognition system with an optionalanti-glare IR polarizer on the lens operating on any of the wavelengthsthat is provided by the IR LEDs will now see different regions of thedisplay as bright or dark, depending on the local polarization state ofthe light.

Accordingly, one or more embodiments of the present invention overcomesone or more of the shortcomings of the known prior art.

For example, in one embodiment, an infrared light readable liquidcrystal display system comprises a liquid crystal display comprising aliquid crystal display cell comprising: a liquid crystal layer tocontrol the polarization state of visible light and infrared light, afront substrate, a rear substrate, and wherein the liquid crystal layeris located between the front substrate and the rear substrate; a visiblelight front polarizer, wherein the visible light front polarizer istransparent to infrared light; and a reflective rear polarizer topolarize visible light and infrared light; and an illumination unitcomprising a plurality of light sources, wherein at least one of theplurality of light sources emits infrared light.

In this embodiment, the infrared light readable liquid crystal displaysystem can further comprise: wherein the liquid crystal display is aninfrared transmissive liquid crystal display, and wherein theillumination unit is a backlight; wherein the backlight furthercomprises an absorber to absorb extra light; wherein the infraredtransmissive liquid crystal display further comprises a visibly opaquelayer transparent to infrared light and the backlight further comprisesa reflector, wherein the reflector reflects infrared light; wherein thebacklight further comprises a polarization conserving lightguide, and aninfrared polarizer, wherein the infrared polarizer is located betweenthe plurality of light sources and the polarization conservinglightguide; wherein the visibly opaque layer is transparent to infraredlight, and wherein the visibly opaque layer appears black in visiblelight; wherein the visibly opaque layer is transparent to infraredlight, and wherein the visibly opaque layer is non-black in visiblelight; wherein the plurality of light sources comprises at least oneinfrared emitting light source and at least one visible light emittinglight source; wherein the liquid crystal display is a reflective liquidcrystal display, and wherein the illumination unit is a front light;wherein the front light is located between the visible light frontpolarizer and the liquid crystal display cell; wherein the front lightis in front of the visible light front polarizer from the perspective ofan observer; wherein the front light further comprises a polarizationconserving lightguide and an infrared capable polarizer, and wherein theinfrared capable polarizer is located between the illumination unit andthe polarization conserving lightguide; wherein the plurality of lightsources comprises at least one infrared emitting light source and atleast one visible light emitting light source.

In another example embodiment, an infrared light readable liquid crystaldisplay system for an electronic license plate comprises a liquidcrystal display comprising a liquid crystal display cell comprising aliquid crystal layer to control the polarization state of visible lightand infrared light, a front substrate, a rear substrate, and wherein theliquid crystal layer is located between the front substrate and the rearsubstrate; a visible light front polarizer, wherein the visible lightfront polarizer is transparent to infrared light and a reflective rearpolarizer to polarize visible light and infrared light; an illuminationunit comprising a plurality of light sources wherein at least one of theplurality of light sources emits infrared light; a plurality of lightsensors wherein at least one of the plurality of light sensors issensitive to infrared light; and an electronic circuit capable ofdriving the at least one of the plurality of light sources that emitsinfrared light.

In this embodiment, the infrared light readable liquid crystal displaysystem for an electronic license plate further comprising amicrocontroller to receive an input from the at least one of theplurality of light sensors sensitive to infrared light, and whereinbased on the input the microcontroller controls the at least one of theplurality of light sources that emits infrared light; further comprisinga separate circuit to receive an input from the at least one of theplurality of light sensors sensitive to infrared light, and whereinbased on the input the separate circuit controls the at least one of theplurality of light sources that emits infrared light; wherein theplurality of light sources comprises at least one infrared emittinglight source and at least one visible light emitting light source; andwherein the liquid crystal display maintains a stable visible imagewithout being refreshed more than once per second.

In another example embodiment, a method of operating an infrared lightreadable liquid crystal display system comprises providing a liquidcrystal display comprising a liquid crystal display cell comprising aliquid crystal layer to control the polarization state of visible lightand infrared light, a front substrate, a rear substrate, and wherein theliquid crystal layer is located between the front substrate and the rearsubstrate; a visible light front polarizer, wherein the visible lightfront polarizer is transparent to infrared light, and a reflective rearpolarizer to polarize visible light and infrared light; providing anillumination unit comprising a plurality of light sources, wherein atleast one of the plurality of light sources emits infrared light; andcontrolling the plurality of light sources based on an externalstimulus.

In another example embodiment, a method of operating an infrared lightreadable liquid crystal display system for an electronic license platecomprises providing a liquid crystal display comprising a liquid crystaldisplay cell comprising a liquid crystal layer to control thepolarization state of visible light and infrared light, a frontsubstrate, a rear substrate, and wherein the liquid crystal layer islocated between the front substrate and the rear substrate; a visiblelight front polarizer, wherein the visible light front polarizer istransparent to infrared light; and a reflective rear polarizer topolarize visible light and infrared light; providing an illuminationunit comprising a plurality of light sources, wherein at least one ofthe plurality of light sources emits infrared light; providing aplurality of light sensors wherein at least one of the plurality oflight sensors is sensitive to infrared light; providing an electroniccircuit capable of driving the at least one light source which emitsinfrared light; and controlling the plurality of light sources with theelectronic circuit based on illumination conditions from the pluralityof light sensors.

Other objects, features, and advantages of the present invention willbecome apparent upon consideration of the following detailed descriptionand the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a typical prior art backlight as used with transmissive ortransflective displays.

FIG. 2 shows a typical prior art front light as used in conjunction withreflective displays.

FIG. 3 shows a liquid crystal display with a backlight according to U.S.Pat. No. 9,190,004.

FIG. 4 shows prior art in accordance with U.S. Pat. No. 9,229,268, whichis an improvement of U.S. Pat. No. 9,190,004 as it eliminates theundesirable mirror-like appearance.

FIG. 5A shows one example embodiment of the invention consisting of aninfrared transmissive LCD display with a reflective rear polarizer and abacklight backed by an absorber.

FIG. 5B shows one example embodiment of the invention consisting of aninfrared transmissive LCD display with a reflective rear polarizer and abacklight backed by a mirror.

FIG. 6 shows one example embodiment of the invention consisting of aninfrared transmissive LCD display with polarization conserving backlightand absorptive visible light rear polarizer.

FIG. 7 shows one example embodiment of the invention consisting of areflective LCD display with non-polarized IR front light, reflectiverear polarizer, and IR analyzer on the camera.

FIG. 8 shows one example embodiment of the invention consisting of areflective LCD display with polarization conserving front light andreflective rear polarizer.

FIG. 9 shows one example embodiment of the invention consisting of adigital license plate display with reflective LCD, front light andcombined IR/visible light illumination.

FIG. 10 shows one example embodiment of the invention consisting of adisplay system in the form of a block diagram.

FIG. 11 shows one example embodiment of the invention consisting of analternative layout of display system in the form of a block diagram.

FIG. 12 shows an example circuit consisting of an IR sensing board andLED driving board that can be used to detect light and drive the IR LEDswithout involvement of a microcontroller.

DETAILED DESCRIPTION OF THE INVENTION

The following is a detailed description of embodiments to illustrate theprinciples of the invention. The embodiments are provided to illustrateaspects of the invention, but the invention is not limited to anyembodiment. The scope of the invention encompasses numerousalternatives, modifications, and equivalents. The scope of the inventionis limited only by the claims.

While numerous specific details are set forth in the followingdescription to provide a thorough understanding of the invention, theinvention may be practiced according to the claims without some or allof these specific details.

Various embodiments will be described in detail with reference to theaccompanying drawings. Wherever possible, the same reference numbers areused throughout the drawings to refer to the same or like parts.References made to particular examples and implementations are forillustrative purposes and are not intended to limit the scope of theclaims.

Background and Prior Art

FIG. 1 shows a typical prior art backlight 100 as used with transmissiveor transflective display 110. The backlight 100 is placed behind thedisplay 110 from a viewer's perspective. Although, other backlightdesigns and principles exist, here the description shall be given usingthe example of an edge-lit lightguide-based backlight. The backlight 100comprises a lightguide 140, which is typically made from a transparentpolymer such as PMMA, PC or glass.

Lightguide 140 is similar in size with respect to length and widthdimensions to display 110 and serves the purpose of spreading the lightuniformly across the display 110. The lightguide 140 has features thatmay be in the bulk or on the surface. Such features in the surface maybe dimples or prisms, in the bulk they may be randomly or regularlydistributed alternate materials. The features are designed as to causethe light 150 to leave the lightguide 140 in a defined direction andwith a uniform intensity distribution as shown in FIG. 1 .

The features may send the light 150 directly to display 110 or first toa reflector behind the lightguide 140 from observer's 160 perspective,from where the light 150 is reflected towards the display 110.Lightguide 140 may incorporate other optical functions, such asdiffusion or light shaping and directing, or these functions may beadded in separate components (usually sheets of films) placed into theimmediate vicinity of the lightguide 140 or adhered to it.

Illuminators 130 are placed along one or more edges of the lightguide140. In one embodiment, illuminators 130 can be side firing LEDs. It isimportant that the light 150 is effectively coupled into the lightguide140 without excessive waste. This is achieved by the design of theinterface between illuminator 130 and lightguide 140 as well as thedesign of the light sources such that the emitted light 150 exits thelight source in a useful range of angles. Typically, a flexible printedcircuit 120 provides the electrical current to operate the illuminators,which may be arranged electrically in series, parallel, or in parallelgroups of smaller series.

In one embodiment, the LEDs comprising the illuminators 130 may beselected to emit white, red, green, blue, or infrared (IR) light or anycombination thereof. One backlight may have LEDs with multiple,different emission wavelengths.

FIG. 2 shows a typical front light 200 as used in conjunction withreflective displays, such as reflective display 210. The elements offront light 200 are similar to that of backlight 100 and includeflexible printed circuit board 220, illuminators 230, and lightguide240. However, front light 200 is placed in front of the display 210 froman observer's 260 perspective, as compared to backlight 100. Additionalrequirements for front light 200 are that the light 150 must beexclusively directed towards the display 210 and that the front light200 must allow for a clear, sharp, and accurate color display image.

FIG. 3 shows a liquid crystal display 300 with backlight 310 accordingto U.S. Pat. No. 9,190,004. The backlight 310 is arranged behind thedisplay 300. The backlight 310 comprises LED light source 320, alightguide plate 340, a reflector 350 behind the lightguide plate 340,and optional light shaping films 330, located between the lightguideplate 340 and the display 300. The display 300 comprises a frontpolarizer 360, an LCD cell 370, and a rear polarizer 380. The LCD cell370 also comprises a liquid crystal layer 390 interspersed between afront substrate 392 and a rear substrate 394.

U.S. Pat. No. 9,190,004 teaches that the LED light source 320 may emitvisible and/or IR light and that the front polarizer 360 and rearpolarizer 380 must be able to polarize both visible and infrared lightin order for the display 300 to have contrast in the visible andinfrared region of the electromagnetic spectrum. Typical liquid crystaldisplay polarizers 360 and 380 work only in the visible range, wherethey absorb one polarization while transmitting the other polarization.Moreover, adding anisotropic IR absorbing chromophores to visible lightpolarizers would be required, and it is not disclosed how this can bedone nor that such additional chromophores would reduce the visiblelight transmission and thus lead to a much dimmer display.

U.S. Pat. No. 9,191,004 teaches that certain reflective polarizers canbe used such as wire grid polarizer sheets, birefringent polarizersheets, or cholesteric liquid crystal polarizer sheets, which work bothin the visible and IR region. The disadvantage of such polarizers isthat they reflect, not absorb the unwanted polarization. While that canbe dealt with in the rear polarizer 380, it is undesirable in frontpolarizer 360 as it gives the display a mirror-like appearance. Brightimage areas in such a display appear bright, while dark image areasappear like a metallic mirror. An observer will see both the displayimage and a reflected scene superimposed onto each other.

FIG. 4 shows prior art in accordance with U.S. Pat. No. 9,229,268, whichis an improvement of U.S. Pat. No. 9,190,004 as it eliminates theundesirable mirrorlike appearance.

The backlight 310 again comprises LED light source 320, a lightguideplate 340, a reflector 350 behind the lightguide plate 340, and optionallight shaping films 330, located between the lightguide plate 340 andthe display 400.

Backlight 310 is placed behind liquid crystal display 400 comprising theLCD cell 370, a sequential stack of two front polarizers comprisingvisible light front polarizer 460 and IR light front polarizer 465, anda sequential stack of two rear polarizers comprising visible light rearpolarizer 480 and IR light rear polarizer 485. The LCD cell 370 alsocomprises a liquid crystal layer 390 interspersed between a frontsubstrate 392 and a rear substrate 394.

In one embodiment, one each of the front polarizers 460 and 465, and therear polarizers 480 and 485 is a reflective type, such as a wire gridpolarizer sheet, a birefringent polarizer sheet, or a cholesteric liquidcrystal polarizer sheet, which work both in the visible and IR region.The other one of the front polarizers 460 and 465 and the rearpolarizers 480 and 485 is a standard liquid crystal display polarizersheet, which work only in the visible range by absorbing onepolarization, while transmitting the other polarization. This worksbecause the standard absorbing polarizer sheets are transparent for bothpolarization states in the IR region.

Hence, in the infrared region the display 400 still is a mirror-likedisplay, while in the visible region the display looks as expected withblack and bright image areas. A human observer will not see thereflected infrared light, while infrared equipment used to view thedisplay has to be arranged and designed such that the reflected unwantedpolarization is not detrimental to the image quality or function of theIR system, such as by using an optical pattern or character recognitionsystem. Using the two front polarizers 460 and 465 and the two rearpolarizers 480 and 485, rather than one of each, adds two costlyelements to the display 400 and since both types of polarizers are not100% transmissive in the visible region of the electromagnetic spectrum,the brightness of the display is diminished by the second polarizer,which is unnecessary for visible light.

In addition, neither U.S. Pat. No. 9,190,004 nor U.S. Pat. No. 9,229,268allow the use of a reflective display, which cannot be operated with abacklight. Neither patent teaches the use of a front light, but if theproposed structure were illuminated with a front light and backed withthe necessary reflector, the display would be very dim as the lightwould travel through a sequence of eight polarizer layers, eachabsorbing a significant portion of the light.

Thus, this disclosure describes systems and methods of IR readabletransmissive and reflective displays without a mirror appearance andwithout the unwanted dimming of the display due to sequential stacks ofpolarizers.

Infrared Transmissive LCD Display 500A and B with a Reflective RearPolarizer 510 and a Non-Polarized Infrared Backlight 505A and B

FIG. 5A shows one example embodiment of this invention consisting of aninfrared transmissive type LCD display 500A with backlight 505A. As aninfrared transmissive type display, LCD display 500A is generallytransmissive to infrared light and reflective of ambient visible light.

Backlight 505A comprises light sources such as LEDs 515, emittingvisible and infrared light, a lightguide 520, an absorber 525 placedbehind lightguide 520 from an observer's 530 vantage point, and in oneembodiment light directing and diffusing films 535 in front of thelightguide 520 and behind LCD display 500A. Optional light directing anddiffusing films 535 may include optical films such as retardation films,compensation films, and other light management films that optimize theperformance of the device.

LCD display 500A further comprises an LCD cell 540 between a visiblelight front polarizer 545 and IR capable reflective rear polarizer 510.LCD cell 540 comprises a liquid crystal layer 550 interspersed between afront substrate 555 and a rear substrate 560.

The front polarizer 545 is of an absorptive type, which absorbs visiblelight of the unwanted polarization, while transmitting visible light ofthe desired polarization as well as all infrared light irrespective ofpolarization. The rear polarizer 510 is a reflective type such as a wiregrid polarizer sheet, a birefringent polarizer sheet or a cholestericliquid crystal polarizer sheet, which work both in the visible and IRregions.

An infrared sensitive image capture or recording device such as camera570 is directed towards LCD display 500A. In one embodiment, camera 570comprises an infrared camera. Camera 570 comprises a lens 575 to focusthe display image onto the sensor element (not shown) inside the camera570. It also comprises an IR analyzer 580 to avoid glare for reflectivesurfaces like the polarizers used in photographic equipment to reduceglare, only with its function optimized for IR wavelengths.

Also directed towards LCD display 500A is observer 530 viewing LCDdisplay 500A via reflection of light from visible environmental lightsource 590. Environmental light source 590 may be diffuse daylight,direct sunlight, room light, light from a dedicated illumination sourceor similar.

In FIGS. 5-11 , unpolarized light 592 is shown as arrows with a solidline, a dotted line illustrates a first polarization 594, such as linears-polarization or circular l-polarization, and a dashed line illustratesthe second orthogonal to the first polarization 596, such as linearp-polarization or circular r-polarization.

In visible light observation of LCD display 500A, unpolarized light 592from the environmental light source 590 enters LCD display 500A. Theportion of the unpolarized light 592 with the undesired polarization isabsorbed in front polarizer 545. Light of the desired polarization istransmitted through front polarizer 545 into LCD display 500A, where inthe liquid crystal layer 550 the light either retains its polarization596 or has its polarization morphed or changed into the orthogonalpolarization 594, depending on the state of the liquid crystal layer550.

Unchanged light is transmitted through reflective rear polarizer 510 andbacklight 505A until it gets absorbed in absorber 525. The correspondingdisplay area looks black or dark to the observer 530. Light with achanged polarization state is reflected by the rear polarizer 510 andchanged back to its original polarization state in the liquid crystallayer 550, and therefore has the correct polarization state to pass thefront polarizer 545 and then travels to the observer 530. Thecorresponding area of LCD display 500A appears bright to the observer530. For visible observation of light from environmental light source590, backlight 505A is not required, however, absorber 525 must beprovided.

For infrared observation, the IR LEDs 515 are activated. Unpolarized IRlight from IR LEDs 515 spreads through lightguide 520 and illuminatesLCD display 500A uniformly from behind. After passing the light shapingand diffusing films 535, the light is polarized in reflective rearpolarizer 510 as only one polarization state is transmitted, while theother polarization state is reflected. The reflected portion of thelight is returned into lightguide 520 and may get absorbed in absorber525.

The polarized light travels through liquid crystal layer 550 anddepending on the alignment of the liquid crystals in liquid crystallayer 550, the light retains its polarization or changes to anotherpolarization state. ‘Bright’ and ‘Dark’ areas of the image emit the sameamount of light but with different polarization states. One of thesestates can pass IR analyzer 580 of camera 570 while the otherpolarization is rejected.

Therefore, bright and dark areas are projected by lens 575 onto theimage sensor inside camera 570, corresponding to the polarization stateemitting from the respective areas of the display. If desired, thecontrast of the image can electronically be inverted before imageanalysis or before displaying the image on LCD display 500A. Becauseonly one polarizer is used on either side of LCD display 500A there isno additional cost, thickness, and undue dimming of the brightness ofLCD display 500A. Because the front polarizer 545 is an absorbing type,there is no mirror-like image display surface.

In an alternative embodiment, backlight 505A can be used in conjunctionwith visible light from LEDs 515. In this embodiment, the visible lightimage using backlight 505A has inverted contrast. Such a displayarrangement may use a visible light sensor (not shown) and activatebacklight 505A while simultaneously electronically reversing thecontrast of the image, so that the observer sees the proper contrast(twice inverted).

It should be clear to those skilled in the art that LCD display 500A canbe operated with the transmission axes of front polarizer 545 and rearpolarizer 510 either orthogonal (crossed) or parallel and with theliquid crystal layer 550 being arranged to retain the polarizationeither when powered or not or either in one or another stable state.This leads to several possible alternative embodiments often referred toas normally white and normally black with direct or inverse contrast.

FIG. 5B shows an alternative embodiment of LCD display 500A shown inFIG. 5A. In FIG. 5B, backlight 505B has reflector 527 as the elementfurthest from observer 530, instead of absorber 525 in backlight 505A.In this embodiment, LCD display 500B has visibly opaque (black) layer526 added to LCD display 500B near backlight 505B. Visibly opaque(black) layer 526 is transparent for infrared light but absorbs visiblelight. Visibly opaque (black) layer 526 can be printed with special dyessuch as Epolight dyes, or it can be formed by a visible light absorptivepolarizer with its transmission axis orthogonal to (crossed) that of thereflective rear polarizer 510. In other embodiments, visibly opaque(black) layer 526 can comprise other colors such as a blue opaque layerthat transmits IR which leads to an embodiment of LCD display 500B witha blue and white contrast rather than a black and white contrast.

LCD display 500B has the advantage of reduced parallax shadow in visiblelight operation as absorber 525 is closer to liquid crystal layer 550.In infrared operation, the reflected polarization of the infrared lightat the rear polarizer 510 is recycled in the backlight 505B. Thisincreases the efficiency of backlight 505B.

Infrared Transmissive LCD Display 600 with Absorptive Visible Light RearPolarizer 610 and Polarized Infrared Backlight 605

FIG. 6 shows another embodiment of the invention. Infrared transmissivetype LCD display 600 is similar to LCD display 500A except reflectiverear polarizer 510 has been replaced by absorptive visible light rearpolarizer 610. Backlight 605 is similar to backlight 505B except the IRpolarizer 612 is added between the IR LEDs 515 and lightguide 620. Inthis embodiment, lightguide 620 is polarization conserving.

For visible light observation, unpolarized light 592 from environmentallight source 590 enters LCD display 600. The portion of the unpolarizedlight 592 with the undesired polarization is absorbed in front polarizer645. Light of the desired polarization is transmitted into LCD display600, where, in the liquid crystal layer 550, the light either retainsits polarization 596 or has its polarization morphed or changed intoorthogonal polarization 594, depending on the alignment of the liquidcrystals in liquid crystal layer 550. Unchanged light is absorbed inrear polarizer 610. The corresponding area of LCD display 600 looksblack to observer 530.

Light with a changed polarization state is transmitted through rearpolarizer 610 and backlight 605 and is reflected by reflector 527. Thereflected light passes through rear polarizer 610 and is changed back toits original polarization state in the liquid crystal layer 550 andtherefore has the correct polarization state to pass through frontpolarizer 545 and then travel to the observer 530. The correspondingarea of LCD display 600 appears bright to observer 530. For visibleobservation with light from environment light source 590, backlight 605is not required, but reflector 527 must be provided.

In an alternative embodiment, backlight 605 can be used with optionalvisible light sources (not shown) if there is insufficient environmentallight. In this case, unpolarized light 592 exiting lightguide 620 ispolarized by rear polarizer 610 and remains unchanged or has itspolarization state changed in the liquid crystal layer 550, depending onthe orientation of the liquid crystals. Light with a changedpolarization state passes through front polarizer 545 and reaches theobserver 530. The corresponding image area of LCD display 600 looksbright to observer 530. Light with an unchanged polarization state getsabsorbed in the front polarizer 545. The corresponding image area of LCDdisplay 600 looks dark to observer 530. This arrangement does not causea contrast inversion.

For infrared observation, IR LEDs 515 are activated. Unpolarized IRlight from IR LEDs 515 is polarized with IR polarizer 612 beforeentering lightguide 620. Polarized IR light spreads through lightguide620 and illuminates LCD display 600 uniformly from behind.

After passing the optional light shaping and diffusing films 535, thelight passes rear polarizer 610 unchanged as this polarizer type appearstransparent to IR light. The polarized light travels through liquidcrystal layer 550 where, depending on the alignment of the liquidcrystals in liquid crystal layer 550, the light retains its polarizationor changes to another polarization state. ‘Bright’ and ‘Dark’ areas ofthe image emit the same amount of light but with different polarizationstates. One of these states can pass IR analyzer 580 of camera 570,while the other polarization is rejected.

Therefore, bright and dark areas are projected by lens 575 onto theimage sensor inside camera 570, corresponding to the polarization stateemitting from the respective areas of LCD display 600. If desired, thecontrast of the image can electronically be inverted before imageanalysis or before displaying the image on LCD display 600. Since onlyone polarizer is used on either side of LCD display 600 there is noadditional cost, thickness, and undue dimming of the display brightness.Because front polarizer 545 is an absorbing type, there is nomirror-like image display surface for LCD display 600.

Those skilled in the art will appreciate that alternative embodimentshave equivalents with parallel and crossed polarizers, and differentliquid crystal director configurations, some of which may be more orless advantageous.

Reflective LCD Display 700 with Reflective Rear Polarizer 510 and withNon-Polarized IR Front Light 705

FIG. 7 illustrates another embodiment of the invention, based on areflective type LCD display 700. A front light 705, comprising LEDilluminators 515 and a light guide 720, is placed between the frontpolarizer 545 and LCD cell 740. The front polarizer 545 is absorptive,which works for visible light, but appears transparent to infraredlight. Rear polarizer 510 is reflective and works for both visible andinfrared light. Located behind the rear polarizer 510 is an absorber525.

For visible light observation, unpolarized light 592 from environmentallight source 590 enters LCD display 700. The portion of the light withthe undesired polarization is absorbed in front polarizer 545. Light ofthe desired polarization 596 is transmitted through front polarizer 545and front lightguide 720 and into LCD display 700, where, in the liquidcrystal layer 750, it either retains its polarization or has itspolarization morphed or changed into the orthogonal polarization 594,depending on the alignment of the liquid crystals in liquid crystallayer 750.

Unchanged light 596 is transmitted through reflective rear polarizer 510and gets absorbed in absorber 525. The corresponding area of LCD display700 looks dark to observer 530. The observer 530 ‘sees’ the blackabsorber 525. Light with a changed polarization state 594 is reflectedby rear polarizer 510, changed back to its original polarization state596 in liquid crystal layer 750, and therefore has the correctpolarization state to pass through front polarizer 545 and then travelto observer 530. The corresponding area of LCD display 700 appearsbright to observer 530. For visible observation with light fromenvironment light source 590, front light 705 is not required.

In an alternative embodiment, front light 705 can be used with optionalvisible light sources (not shown) and a visible light polarizer (notshown) between the visible light sources and lightguide 720 if there isinsufficient light from environmental light source 590. In this case,polarized light exiting the lightguide 720 takes the same path asenvironmental light after passing through front polarizer 745.

In another alternative embodiment, front light 705 can be positioned infront of front polarizer 745. In this case, the visible light polarizer(not shown) between visible light source (not shown) and lightguide 720is not necessary.

For infrared observation, IR LEDs 515 are activated. Unpolarized IRlight from the light source 590 spreads through lightguide 720 andilluminates LCD display 700 uniformly from the front. Because frontpolarizer 545 is transparent to IR light, in a modification of thisembodiment, front light 705 can also be in front of front polarizer 545from the vantage point of observer 530. The light exiting lightguide 720towards LCD display 700 travels unchanged through liquid crystal layer750 to rear polarizer 510, where light of one polarization is reflectedinto LCD display 700 while light of the other polarization istransmitted through reflective rear polarizer 510 and is absorbed inabsorber 525.

The polarized light reflected into LCD display 700 travels through theliquid crystal layer 750 where, depending on the alignment of the liquidcrystal in layer 750, the light retains its polarization or changes toanother polarization state. The front light 705 and front polarizer 545are transparent to IR light. ‘Bright’ and ‘Dark’ areas of the image emitthe same amount of light but with different polarization states. One ofthese states can pass the IR analyzer 580 of camera 570 while the otherpolarization is rejected.

Therefore, bright and dark areas are projected by lens 575 onto theimage sensor inside camera 570, corresponding to the polarization stateemitting from the respective areas of the display 700. If desired, thecontrast of the image can electronically be inverted before imageanalysis or before displaying the image on LCD display 700. Since onlyone polarizer is used on either side of LCD display 600 there is noadditional cost, thickness, and undue dimming of the display brightness.Because the front polarizer 545 is an absorbing type, there is nomirror-like image display surface for LCD display 600.

Reflective LCD Display 800 with Reflective Rear Polarizer 510 andPolarized Infrared Front Light 805

FIG. 8 illustrates another embodiment of the invention. Reflective typeLCD display 800 is similar to LCD display 700 except IR polarizer 812 isplaced between the IR LEDs 515 and front lightguide 820. Frontlightguide 820 must be polarization maintaining.

The light path and functional principle for visible light observationfor LCD display 800 is the same as for LCD display 700. For infraredobservation, IR LEDs 515 are activated. Unpolarized IR light from IRLEDs 515 is polarized with IR polarizers 812 at the light source.Polarized IR light spreads through the polarization maintaininglightguide 820 and illuminates the LCD display 800 uniformly from thefront. In another modification of this embodiment, because the frontpolarizer 545 is transparent to IR light, the front light 805 can alsobe in front of the front polarizer 545.

The polarized light exiting lightguide 820 into LCD display 800 travelsunchanged to liquid crystal layer 750 where, depending on theorientation of the liquid crystal in liquid crystal layer 750, the lightretains its polarization or changes to another polarization state.Unchanged light 596 is transmitted through reflective rear polarizer 510and is absorbed in absorber 525. The corresponding area of LCD display800 is imaged via lens 575 as a black area onto the sensor inside camera570 since no light is traveling to the camera.

Light with a changed polarization state 594 is reflected by rearpolarizer 510 and is changed back to its original polarization state 596upon passing liquid crystal layer 750 a second time. Front polarizer 545is transparent to IR light, so the light continues to camera 570. Thecorresponding area of LCD display 800 is imaged via lens 575 as a brightarea onto the sensor inside camera 570. In this configuration, IRanalyzer 580 is not required. If the camera system has an antiglare IRpolarizer 580, the polarization directions of LCD display 800 must beconfigured such that the light 596 traveling to camera 570 issubstantially vertically polarized. This ensures the light 596 can passthe antiglare IR polarizer 580, which blocks horizontally polarizedlight.

Digital License Plate with Reflective LCD, Front Light and CombinedInfrared and Visible Light Illumination

FIG. 9 illustrates display system 900, which in one embodiment is thedisplay system of a digital license plate that requires visiblereadability and IR optical pattern or character recognition. In thisexample, for display system 900, LCD display 800 is placed into housing905. However, in other embodiments, LCD display 500, LCD display 600, orLCD display 700 could also be used in place of LCD display 800 indisplay system 900. The housing 905 comprises a front lens 950, an IRlight sensor 955 sensitive only to IR light, and a daylight sensor 960sensitive only to visible light wavelengths. In addition, in oneembodiment, additional optional visible light LEDs 915 may be added.

Digital license plates (not shown) must be readable by automated licenseplate recognition (ALPR) camera system 975. ALRP camera system 975comprises one or more ALRP cameras 970, working with visible and IRlight and infrared light illuminators 980, next to the ALRP camera 970.ALRP camera system 975 is optimized to read retroreflective licenseplates or license plates with diffuse, Lambertian reflectance and isnecessarily placed above or to the side of the roadway or on anothervehicle.

The illumination from IR illuminator 980 is coaxial with ALPR camera970. Such light, however, is reflected by a digital license platesubstantially away from ALPR camera 970, rather than back towards ALPRcamera 970. This necessitates the use of internal IR illumination of thelicense plate.

Display system 900 has suitable electronic circuits (as shown in FIG. 10) that activate IR LEDs 515 when IR light sensor 955 senses a rapidchange in IR intensity, which occurs, for example, if LCD display 800 isbeing flashed with ALPR camera 970 or if a vehicle drives into an IRflood illumination zone. Such electronic circuits may be based on amicrocontroller unit and firmware determining when to turn on IR LEDs515.

FIG. 10 shows display system 900 in the form of block diagram 1000consisting of microcontroller unit (MCU) 1040 connected to battery 1010,day light sensor 960, IR sensor 955, optional peripherals 1050, LCDdisplay 800, and illuminator unit 1070 consisting of visible light LEDs915, second light source 1082, IR LEDs 515, and fourth light source1086. Illuminator unit 1070 is used to illuminate display 800. Inalternative embodiments, additional light sources may also be used.

Alternatively, for lower power consumption and faster response, suchcircuits may connect the IR light sensor 955 to an operationalamplifier, which drives a current source for IR LEDs 515, causing IRLEDs 515 to flash back in sync with being flashed by IR light withoutthe digital license plate and its microcontroller system having to wakeup from a low power state.

FIG. 11 shows such an alternative layout of display system 900 in theform of block diagram 1100 consisting of MCU 1040 connected only tobattery 1010, optional peripherals 1050, and LCD display 800. Lightsensor circuit 1110 is connected directly to the battery and controlsilluminator unit 1070, visible light LEDs 915, and IR LEDs 515.Illuminator unit 1070 is used to illuminate display 800.

FIG. 12 shows an example of light sensor circuit 1110 that can be usedto detect light and drive the IR LEDs 515 without involvement of MCU1040, consisting of IR sensing board 1210 and LED driving board 1220.

Turning back to FIG. 9 , the internal IR illumination of display system900 represents an active response to interrogation, directed towardsALPR camera 970 resulting in a brighter image, lower signal-to-noiseratio and hence a better accuracy of the optical pattern or characterrecognition. The IR wavelength of the interrogating system and the IRwavelength of the active response are independent. The IR wavelength ofinternal IR LEDs 515 illuminating LCD display 800 can be chosen toachieve the best possible contrast and accuracy in the IR image captureand recording system.

The reflective LCD display 800 may be a bi-stable or multi-stable LCDdue to the low power requirements of such displays compared to displaysrequiring constant updating. In one embodiment, the liquid crystaldisplay maintains a stable visible image without being refreshed morethan once per second. One such bistable LCD type may be amemory-in-pixel LCD, another may be a bistable nematic LCD known asBinem, or a bistable nematic display known as ZBD.

The LCD display 800 can work with a reflective rear polarizer 510, suchas a multilayer polymer stack available from 3M™ known as DBEF, a wiregrid polarizer such as WGF from Nagase, or similar. Such reflectivepolarizers have usable contrast from about 380 nm to greater than 850nm.

Front lightguide 820 may be located on top or below front polarizer 845.It is illuminated from the edge with optional white light LEDs 915 fornight visibility, controlled by daylight sensor 960, and with aplurality of IR LEDs 515 selected for a desired wavelength or multipledesired wavelengths depending on the requirements of the location wheresuch a license plate is issued. Automated license plate recognitionsystems operate at specific infrared wavelengths, such as 740 nm, 850nm, 940 nm and others. For example, IR LED's may comprise several 740 nmand several 850 nm LEDs if that matches the requirement. One of skill inthe art would understand that other combinations are possible as well.

Polarizer 812 may be a dye type polarizer with dye selected for infraredoperation and it is not required to have good transmission orpolarization efficiency in the visible spectrum as no visible light isrequired to pass through it. Another suitable type of polarizer may be awire grid type polarizer or multi-layer stack polarizer as suchpolarizers are simpler and easier to produce at a lower cost than wiregrip polarizers for the visible range.

Polarization preserving lightguide 820 may be made from transparentpolymers, glass, or a combination of different transparent materials andmay be coated with materials of a different refractive index.

Also shown in FIG. 9 is the light path 990 for observer 530 in nightmode. If the daylight sensor 960 detects a dark environment, it mayactivate the visible light LEDs 915. Unpolarized light from visiblelight LEDs 915 travels through the lightguide 820 from where it isdirected uniformly towards LCD display 800. Because the light isunpolarized it passes through the display unchanged.

Part of the light is reflected at rear polarizer 510, while light of theundesirable polarization passes through rear polarizer 510 and isabsorbed by absorber 525. The reflected polarized light will eitherremain unchanged or its polarization will be changed by the liquidcrystal layer 850, depending on the orientation of the liquid crystalsin liquid crystal layer 750.

Light with an unchanged polarization state will be absorbed by frontpolarizer 545. The corresponding image areas of LCD display 800 appeardark to observer 530. Light with a change in polarization state passesfront polarizer 545 and reaches observer 530. Corresponding displayareas of LCD display 800 appear bright. In an alternative embodiment,front light 805 can also be placed in front of front polarizer 845. Inthis case visible light exiting lightguide 820 will first be polarizedby the absorptive front polarizer 845 before passing to the display inan analogous fashion. In yet another alternative embodiment, a visiblelight polarizer can be placed between visible light LEDs 915 andlightguide 820.

While the invention has been specifically described in connection withcertain specific embodiments thereof, it is to be understood that thisis by way of illustration and not of limitation. Reasonable variationsand modifications are possible within the scope of the foregoingdisclosure and drawings without departing from the spirit of theinvention.

What is claimed is:
 1. An infrared light readable liquid crystal displaysystem comprising: a liquid crystal display comprising: a liquid crystaldisplay cell comprising: a liquid crystal layer to control thepolarization state of visible light and infrared light; a frontsubstrate; a rear substrate; and wherein the liquid crystal layer islocated between the front substrate and the rear substrate; a visiblelight front polarizer, wherein the visible light front polarizer istransparent to infrared light; a reflective rear polarizer to polarizevisible light and infrared light; an illumination unit comprising aplurality of light sources, wherein at least one of the plurality oflight sources emits infrared light; wherein the liquid crystal displayis a reflective liquid crystal display; and wherein the illuminationunit is a front light.
 2. The infrared light readable liquid crystaldisplay system of claim 1 wherein the front light is located between thevisible light front polarizer and the liquid crystal display cell. 3.The infrared light readable liquid crystal display system of claim 1wherein the front light is in front of the visible light front polarizerfrom the perspective of an observer.
 4. The infrared light readableliquid crystal display system of claim 1 wherein the front light furthercomprises a polarization conserving lightguide and an infrared capablepolarizer, and wherein the infrared capable polarizer is located betweenthe illumination unit and the polarization conserving lightguide.
 5. Theinfrared light readable liquid crystal display system of claim 1 whereinthe plurality of light sources comprises at least one infrared emittinglight source and at least one visible light emitting light source.